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LMX2572: Question about LMX2572 Locking

Nicky Lim
Genius 3195 points
Part Number: LMX2572
Other Parts Discussed in Thread: LMX2594

Hello,

During LMX2572 test, Locking works unexpected.

Test procedure is as below.

  1. Use DDS output as Ref. input for LMX2572 (Sweep LMX2572 output frequency by sweep Ref. input.)
  2. DDS sweep from LMX2572 Output frequency 1.6GHz by 10KHz steps. ( VCO  parameter is fixed.)
  3. To 10MHz,  LMX2572 output frequency sweep by DDS sweep is normal, and keeps Lock states.
  4. But after 10MHz, Lock states is unlocked, and proceed  F-cal. (When F-cal proceed,  lock time is around 300us. Full Assisted.)
  5. Whe using LMX2594, it works upto 160MHz.

 

Q. Is it normal Locking is kept only upto 10MHz when LMX2572 Ref. input sweep?

Regards,

Nicky

  • 0
    TI__Guru** 107390 points
    Hi Nicky,

    What is the output divider setting? If it is div/4, then the VCO tuning range becomes 40MHz.
    When it is locked, the Vtune voltage is approx. 1.25V. The rails are approx. 0.2V to 2.2V. So if you get it locked and then change the reference clock frequency, Vtune voltage could variate +/-1V before it is unlocked.
    Depending on which VCO core you are using, the Kvco varies between 40MHz/V to 60MHz/V. That means, you can adjust the VCO frequency by the same amount.